JELLYFISHES 103 



trally over the subumbrellar surface to the region of the 

 large circular sheet of muscle that forms a sphincter-like 

 organ midway between the centrally located mouth and 

 the edge of the bell. It is the contraction of this muscular 

 band that reduces the cavity of the bell and, by thus driv- 

 ing the water out of this cavity, forces the animal for- 

 ward. On the relaxation of this sphincter, the bell re- 

 sumes its expanded form in consequence of the elastic 

 action of its gelatinous tissue. The muscle libers in the 

 great majority of coelenterates are like the smooth or 

 non-striped variety in the higher animals, but the sphinc- 

 ters of jellyfishes are exceptional in this respect and are 

 composed of cross-striped fibers. Not only are they thus 

 structurally exceptional, but they are also unlike the or- 

 dinary coelenterate muscle in that they are incapable of 

 long-continued tonic contraction (Jordan, 1912) and in 

 this respect they also resemble the cross-striped muscle 

 of higher forms. 



As already stated, each of the eight marginal bodies 

 of Aurelia consists of a group of sense organs whose 

 cells are a modified part of the superficial epithelium of 

 the region. From the deep ends of these cells nervous 

 prolongations reach out from the given sense organ 

 toward the sphincter. The stretches of nervous tissue 

 that thus emerge from each marginal body are made up 

 not only of the basal prolongations of sense cells, but of 

 many interpolated nerve cells whose form is usually that 

 of a bipolar cell. In Aurelia these stretches of nervous 

 tissue spread out almost at once on the plate-like sphinc- 

 ter, but in Rhisostoma, according to Hesse (1895), the 

 nervous tissue from each of the eight marginal bodies 

 forms a fairly well circumscribed band, which retains its 

 radial course from near the periphery of the bell till it 



